Resistance to anticancer agents by tumor cell populations presents a major barrier to successful cancer chemotherapy today. In vitro exposure of tumor cell populations to cytotoxic natural products such as vinca alkaloids generally elicits the emergence of a multidrug resistance (MDR) phenotype. MDR is defined as resistance against the cytotoxic agent to which cells have been exposed and cross-resistance against diverse cytotoxic agents, many of which are anticancer drugs. The most consistent change detected in MDR cells in vitro is overexpression of P-glycoprotein, a membrane-spanning protein believed to pump anticancer drugs out of cells. The tumor promoter receptor protein kinase C (PKC) is also implicated in MDR. Specific PKC activators, such as 12-0-tetradecanoylphorbol-13-acetate (TPA), can confer a phenotype resembling MDR on drug-sensitive cells, and several PKC inhibitors can partially reverse MDR in vitro. TPA also induces P-glycoprotein phosphorylation, suggesting that PKC may regulate P- glycoprotein function in MDR cells. A direct correlation has been observed between the level of PKC activity and MDR in cultured murine fibrosarcoma UV-2237M cells, suggesting the usefulness of UV-2237M cells in studies aimed at defining the role of PKC in MDR. The proposed studies will establish the basis for elevated PK activity in MDR UV-2237M cells, since it may provide a modus operandi for reversing MDR. The proposed studies will also determine whether PKC activation regulates P-glycoprotein function, by measuring the effects of PKC catalysis on the drug-binding and ATPase activities of P-glycoprotein. Since PKC activation leads to numerous changes in gene expression, its effects on P-glycoprotein expression will be determined. Little is known regarding the role of P-glycoprotein in drug resistance in vivo. In vivo resistance mechanisms may involve environmental factors, intrinsic properties of MDR tumor cells, or both. In the proposed studies, drug-resistant tumors and metastases will be generated using UV-2237M cell lines in syngeneic mice to determine whether P-glycoprotein or PKC expression correlates with drug resistance in tumors and metastatic nodules located in various organ environments in vivo.